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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

390

FATTY SUBSTANCES  (GENERAL METHODS)

by their insolubility in alcohol or aniline, in which solvents tar oils and
resin oils dissolve.

20. Detection and Determination of the Resin

Resin (colophony) 1 is often found mijxed with fatty substances (especi-
ally boiled linseed oil for varnishes and the like), waxes, and particularly
soaps.

1.  Qualitative Investigation.—With neutral fats or oils, fatty acids,
or waxes, 5-10 grams of the substance are heated to boiling for a few moments
with as much 70% alcohol and the alcoholic liquid allowed to cool, filtered
off and evaporated :   the colophony, which is easily soluble in alcohol,
remains as residue and is identifiable by its general characters and by means
of the following reaction (Morawski's) :

A small quantity of colophony, dissolved in 1-2 c.c. of acetic anhydride
and then treated with 1-2 drops of sulphuric acid of D 1-53 (34-7 c.c. of
sulphuric acid of 66° Baume plus 35-7 c.c. of water), gives a transient, violet-
red coloration.

A similar reaction is, however, given by cholesterol when, for instance,
wool fat may be present. In such case the residue from the evaporation
of the alcoholic liquid is taken up in dilute potassium hydroxide solution
(which readily dissolves colophony), the liquid being shaken with ether
(which dissolves cholesterol) and the aqueous alkaline liquid acidified and
the resin acids thus obtained tested by means of Morawski's reaction.

With soap, about 5 grams are dissolved in water and the solution shaken
with ether, the aqueous liquid being acidified and the fatty acids tested by
Morawski's reaction.

2.  Quantitative Investigation.—When mixed with fatty substances
or with soaps, colophony may be determined by Twitchell's method, which
is based on the fact that, in alcoholic solution, the acids of the resin are
not esterified by gaseous hydrogen chloride, whilst fatty acids are readily
converted into ethyl esters under these conditions.   The procedure is as
follows:

The mixture of fats and resin is saponified in the usual way and the
fatty acids then separated by acidifying the soap solution. In the case
of a soap, this is dissolved in water and the solution filtered and then decom-
posed by acid. With mixtures containing unsaponifiable substances it is
necessary, after saponification, to extract the liquid with benzene or petro-
leum ether to remove the unsaponifiable matter, the aqueous solution being
then decomposed with an acid. The fatty and resin acids thus obtained
are well washed and dried and 2-3 grams dissolved in 50 c.c. of absolute
alcohol and dry hydrogen chloride gas passed into the solution kept at
about 10° by immersion in water and ice. The current of gas is stopped
after ij hour, when the saponification is complete. After an hour's rest,
the liquid is diluted with 5 vols. of water and boiled until the esters, mixed
with resin acids, float in a clear layer and the alcohol is theo eliminated.

The resin acids may then be determined either volumetrically or gravi
metrically.

1 For its characters, see Colophony, Vol. II, Chapter IX.elts at 145-150°